Flattening device

ABSTRACT

A flattening device for flattening a metal workpiece includes a frame; a first tightening assembly at a first end of the frame; a second tightening assembly at a second end of the frame; a series of work rollers interposed between the first tightening assembly and the second tightening assembly, wherein the work rollers include a first upper roller and a second upper roller, and a third, lower roller offset and interposed between the first and second upper rollers; wherein the upper and lower rollers accommodate bends in the workpiece of up to five yield stresses. 
     A method of flattening a sheet of metal includes providing a flattening device for flattening a metal workpiece, providing a first pair of pinch rollers at a first end of the flattening device; providing a pair of pinch rollers at a second end of the flattening device; providing a series of work rollers interposed between the first pair of pinch rollers and the second pair of pinch rollers, wherein the work rollers include a first upper work roller and a second upper work roller, and a third, lower work roller offset and interposed between the first and second upper rollers; and accommodating bends in the sheet of up to five yield stresses as the sheet passes between the upper and lower work rollers.

CLAIM OF PRIORITY

This application claims priority from Provisional Application Ser. No. 61/141,860, filed on December 31, 2008, which is hereby incorporated by reference.

BACKGROUND OF THE DISCLOSURE

This disclosure relates to machines used for unfinished or finished metal products to eliminate the effects of bending and warping generally that occur after application of mechanical processes or heat treatment. Distortion commonly produced in unfinished metal products arises during the cooling stage that follows the rolling process and causes bending in the plane perpendicular to that to which the minimum moment of inertia in the unfinished product corresponds. The internal stresses present in metal sheets usually originate in some irregularity in the mechanical characteristics of the rolling operation.

Metal is formed into strip by a process known as rolling, wherein the strip is passed between a pair of work rolls of a rolling mill to reduce its cross-sectional thickness. In the process, the strip is elongated and rolling continues until the strip is reduced to the cross-sectional thickness desired. This rolling process may start with heated billets or slabs of metal, wherein the metal is rolled at a very high temperature, or it may start with previously rolled strip wherein the strip is passed between work rolls in the cold state. In either event, when the strip exits from the mill, it may be convolutedly wrapped to form a coil. When the coil has been formed, curvature of the coil tends to stay with the strip when it is necessary to uncoil the strip for further processing.

Thus, the primary problem with strip coming off of a coil is the curvature which remains with the strip and which varies throughout the entire length of the coil as a function of the radius of any particular portion of the strip while in the coil. Accordingly, the outer wrap of the coil will have less curvature than an inner wrap. It is necessary to remove this curvature so that the strip may be cut accurately and rendered suitable for other manufacturing operations, such as punching, drawing, forming and the like. It is well established that the flatter the strip is prior to a subsequent manufacturing operation, the more accurate and satisfactory will be the end product of that operation.

In addition to strip curvature, other unwanted properties are sometimes impressed upon the strip during hot and/or cold rolling which render the problem of flattening strip much more complex. In order to reduce cross-sectional thickness of the strip during rolling, it is necessary to force the strip between rolls under tremendous pressure whereby the strip essentially becomes a wedge which tends to separate the rolls.

Steel casts can be of a heavy thickness, up to about ten inches thick. The steel is fed through a rolling mill to flatten and make the sheet of metal thinner. The sheet passes back and forth between opposing rolls or through rolls in series to make the sheet thinner and thinner. The sheet may eventually get very long, up to 1,100 feet long and about ¼ inch thick, or about 500 feet long and about ½ inch thick. The sheet, due to its excessive length, may become difficult to manage. As a result, the sheet may be rolled into a coil, and the sheet may eventually be formed into flat plates. The sheet, when it is in the coiled form, may still be hot, soft and pliable. As a result, the sheet may become curved or curled, especially at its edges.

The outside of the sheet may go into compression, and bulges the sheet toward the middle, forming what is commonly called a cross bow.

The other effect that may occur is called coil set, where the sheet is unrolled to uncoil it to make it flat, but the sheet does not remain flat. The sheet is bent past its elastic limit and may be curled or wavy on its edges.

One method of eliminating the coil set is to run the sheet through a leveler, which has multiple rolls. A metal strip product is fed into a roller leveler, typically from a coil. Roller levelers use multiple work rolls to flatten the strip as it passes through the leveler. The path of the strip passes between offset upper and lower work rolls, in effect reverse bending the strip multiple times before the strip exits the leveler.

A typical roller leveler is designed to process a range of strip thicknesses and strip yield strengths. As the strip passes between the work rolls, very high separating forces are generated against the work roll face, yet the work roll diameters are of necessity relatively small; this is to allow the work rolls to bend and to space them close enough to properly work the strip. The work rolls are supported by flights or groups of narrow face back-up rolls. These flights of back-up rolls can be positioned at different elevations across the work roll face. This allows the work rolls to assume a bend or flexed profile. The back-up rolls support the work rolls and prevent them from incurring excessive bending in reaction to the separating forces.

The roller leveler bends the sheet back and forth to minimize shape defects in the metal strip. Roller levelers have the ability to produce a very flat product; however, the metal strip will have internal built up residual stresses.

During subsequent processes such as shearing, cutting and others, these internal tensions cause warping and in particular a lack of flatness. Such defects are chiefly due to differences in length of the metal fibers as well as to internal residual and opposing stresses.

In a roller leveler the work rolls are very slender (in comparison to the flattener rolls), and the work roll face is supported by multiple flights of back-up rolls, all of which can change position (vertically relative to the work roll face). This allows the work roll face to “give,” i.e. flex, or bend, under the load imposed by the metal strip. This flexing action is used to selectively vary the degree of roll penetration (bending of the metal strip around the roll) across the face of the work roll. This feature is used to selectively correct different metal strip defects such as edge wave or center buckle. In the application of the flattener of the present disclosure, there is no need for the work roll flexing because edge wave, center buckle, and so on is removed by the stretch leveler which is downstream of the flattener.

Another method or technique of trying to eliminate cross bow or coil set is to stretch the sheet using a stretcher including clamping jaws. The stretcher clamps and then stretches the sheet past its elastic limit which results in a very flat product. A stretcher may result in more balanced stresses and less tendency for distortion of the metal strip during subsequent cutting operations. A stretcher removes edge wave, center buckle, camber and coil set, but the stretching action does not effectively remove cross bow, and with larger degrees of stretching can actually increase the cross bow defect.

A cutter then is used to cut the sheet into appropriate manageable lengths. A laser or plasma cutter may be used. The cutter may cut the sheets into various shapes or configurations, including square, rectangular, circle or angled. The sheet metal may then be welded together for use in a variety of applications.

Oftentimes, a laser cut of sheet results in a sheet losing balance and becoming warped. The sheet or plate is not flat and is not usable, so it has to be reflattened and machined flat, so that it is usable.

To achieve flatness all the metal fibers must be of the same length, creating new internal stresses which act against and can overcome the existing ones. Flattening devices are commonly used to achieve the desired flatness.

This disclosure relates to an improved flattening device to be used with sheets of metal to eliminate large bends in sheet metal prior to it being fed into other devices such as roller levelers or straighteners.

Thus, there is a need for a flattening device to prepare the sheet to be fed into a leveler or stretcher which eliminates or minimizes cross bow as well as coil set, including large or heavy bends in the sheet.

SUMMARY OF THE DISCLOSURE

This disclosure relates to flatteners. More particularly, it relates to flatteners for eliminating cross bow and coil set from sheet metal.

This disclosure relates to a flattening device to be used with sheets of metal to eliminate large bends in sheet metal prior to it being fed into a roller leveler or straightener.

Specifically, the flattener includes a first tightening device at the entry of the flattener in the form of a first pair of opposed pinch rollers, a group of work rollers for flattening the sheet positioned on opposite sides of the sheet and a second tightening device in the form of a second pair of opposed pinch rollers at the exit of the flattener. The group of work rollers is interposed between the first pair of pinch rollers and the second pair of pinch rollers. Tension is provided by the tightening devices at entry and exit of the flattener, and the pinch rollers maintain the sheet at a constant previously set tension while it is passing through the machine.

The work rollers used for flattening are rotated or moved by passage of the sheet between the rollers which in turn produces friction and so moves or rotates the rollers. The work rollers in turn transmit movement to back up rollers whose function is to sustain the work rollers and prevent any excessive bending. Alternatively, the work rollers can be driven by a separate motorized or mechanical means.

The work rollers preferably have a constant diameter and their contact with the sheet is maintained by the position of the hydraulic cylinders that oppose the bending forces applied to the work rolls by the bending of the sheet. The upper work rolls are positioned to cause an intermesh gap that requires the sheet to bend over the work rolls. The lower work roller can be positioned higher than the straight through path of the sheet. Flatness can be achieved by adjusting the degree of roll intermesh between the upper and the lower work rolls. Adjustment of the flattener is left to the discretion of the operator.

In accordance with one aspect of the disclosure, a flattening device for flattening a metal workpiece, includes a frame; a first tightening assembly at a first end of the frame; a second tightening assembly at a second end of the frame; a series of three work rollers interposed between the first tightening assembly and the second tightening assembly, wherein the work rollers include a first upper roller and a second upper roller, and a third, lower roller offset and interposed between the first and second upper rollers; wherein the upper and lower rollers accommodate bends in the workpiece of up to five yield stresses.

In accordance with another aspect of the disclosure, a method of flattening a sheet of metal includes providing a flattening device for flattening a metal workpiece, providing a first pair of pinch rollers at a first end of the flattening device; providing a pair of pinch rollers at a second end of the flattening device; providing a series of three work rollers interposed between the first pair of pinch rollers and the second pair of pinch rollers, wherein the work rollers include a first upper work roller and a second upper work roller, and a third, lower work roller offset and interposed between the first and second upper rollers; wherein the upper and lower rollers accommodate bends in the sheet of up to five yield stresses as the sheet passes between the upper and lower work rollers.

Another aspect of the flattener of the present disclosure is to remove coil set and cross bow. Typical flatteners do not have a need to remove cross bow, because typically there will be a roller leveler or temper mill downstream of the flattener.

The flattener accomplishes the removal and reversal of cross bow, because relative to conventional flatteners it uses smaller work rolls that use back-up rolls, and it has enough power and roll penetration to achieve five yield strains on the metal strip. Typical flatteners are designed to remove coil set and typically do not have the power or the roll penetration capability to achieve the five yield strains on the sheet (usually limited to a maximum of three yield strains). Also, the typical flattener would have more work rolls than the three in the disclosure.

Another aspect of the disclosure is minimizing the number of bends applied to the metal strip. Conventional flatteners do not have the ability to raise the bottom work rolls above the pass line of the incoming metal strip. In the flattener of the disclosure, the bottom work roll can be raised above the pass line. This contributes to the unique ability of this flattener to reverse cross bow.

Still another aspect of the disclosure is that the flattener can accommodate heavy or large bends in the sheet of material.

Another aspect of the disclosure is the flattener frees the material from its residual stresses.

Another aspect of the disclosure is that the flattener reduces the number of bends in the sheet.

Still another aspect of the disclosure is the flattener reduces or eliminates cross bow and coil set in the sheet.

Yet another aspect of the disclosure is the flattener reduces the number of rolls used to bend the sheet of material and the rolls are oriented in an offset configuration.

Still another aspect of the disclosure is that the flattener is more precise and predictable than existing flatteners.

Other aspects of the disclosure will become apparent upon a reading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof and wherein;

FIG. 1 is a side elevational view in partial cross section for a flattener in accordance with the present disclosure;

FIG. 2 is a front elevational view of the flattener of FIG. 1;

FIG. 3 is an enlarged partial side elevational view in cross section of the flattener of FIG. 1;

FIG. 4 is a cross sectional view through an upper work roll of the flattener of FIG. 1;

FIG. 5 is a view through section A-A of FIG. 4;

FIG. 6 is a cross sectional view through a lower work roll of the flattener of FIG. 1; and

FIGS. 7A and 7B show a top and front elevational view of the flattener of FIG. 1.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to a flattening device. More particularly, it relates to a flattener which accurately and predictably can take one or several heavy bends of a sheet of metal and reduce or eliminate cross bow and coil set in a sheet to result in a substantially flat sheet.

The flattener can be used in conjunction with either a conventional stretcher or a roller leveler, to compensate for and eliminate or minimize cross bow, as well as coil set in a sheet of metal, such as steel.

The present disclosure relates to an improved flattener which is designed to compensate for more extreme or heavy bends in the sheet, depending on the thickness and strength of the material. Heavy bends of up to 4 to 5 yield stresses and 75-80% of material into plastic deformation can be accommodated by the flattener. The sheet of material is typically substantially flat in the middle and wavy or bent on the edges, which results in edges which are longer than the midsection of the sheet.

Furthermore, the sheet when it is rolled has a coil set consistent along the length of the roll. The flattener is used to minimize or substantially eliminate the coil set to feed the sheet into other equipment, such as levelers or straighteners.

The roll configuration of the flattener is unique in that three work rolls or rollers are used which are typically offset from each other and which receive the sheet material which first passes through a first set of opposed pinch rolls or rollers which are closely positioned to form a tight entry for the sheet of material as it enters and exits the pinch rolls. The work rolls include a first, entry roll, a second, exit roll and a third, bottom roll. Each has a back-up roll to add stiffness and compensate for the work rolls receiving stresses from the large bends in the sheet.

Each of the work rollers is independently adjustable to provide an intermesh (i.e. roll gap) between the rollers that forces the sheet to follow a heavy bend path. The roller positions are very precise. The rollers are programmed to be adjustable based on the gauge and strength of the material, and the different bend radii. The programmable positions are determined based on the amount of bending and the radius of the bends, for both positive and negative bends.

One of the primary functions of the flattener is to remove curvature from a piece of metal strip, sheet or plate. A strip is defined to mean a piece of metal which is sufficiently narrow and is rolled sufficiently thin that it can be wrapped into a coil. A sheet is defined as a piece of metal that is, for whatever reason, cut into lengths rather than stored in coiled form. A plate is a piece of metal which is too thick, as a practical matter, to be formed into a coil.

In the case of sheets and plates, the curvature would normally be of a substantially constant radius and the flattener could be of the simplest form to flatten the sheet or plate. For this operation, the flattener would require an upper work roller and a pair of lower work rollers. It will be observed that a sheet moving from right to left is flexed downwardly between an upper work roller and a lower work roller to the right of the upper work roller and then is reverse flexed between the upper work roller and another lower work roller to the left of the upper work roller which removes the simple curvature from the sheet. To remove the curvature from the sheet the upper work roller and lower work rollers must be properly positioned with respect to each other. This positioning will vary depending upon the amount of curvature which must be removed from the sheet. Thus, the upper and lower work rollers are vertically adjustable with respect to each other to increase or decrease the gap between the rollers.

Referring now to the Figures in greater detail, and in particular to FIGS. 1 and 2, therein is shown flattener A comprising a weldment frame 10 having steel side slabs 11, welded to base slab 13 to form the lower half of the frame.

Referring now to FIGS. 1, 2, 3 and 7A and 7B, a flattener assembly A includes a pair of opposing pinch rollers 12, 14 positioned at a first, or entry point 15 of the flattener assembly and a second pair of opposing pinch rollers 16, 18 positioned at a second, or exit point 19 of the flattener. The pinch rolls are each about 12 inches in diameter. The pinch rolls are each positioned to form a tight engagement with a work piece WP (FIG. 1) of metal sheet as the sheet is fed through the pinch rolls. The pinch rolls can move positions or travel, i.e. they can be adjusted vertically and can be positioned in various locations up to about 12 inches apart. The pinch rollers are raised or lowered by hydraulic cylinders 20, 22. The pinch rollers are driven or rotated by any suitable electro-motorized means, such as electro-motors 30, 32 shown in FIGS. 7A and 7B. Motors 30, 32 have arms 50, 52 attached to mounting members 54, 56 of the rollers. Arms 50, 52 can pivot as the rolls travel up and down vertically. The pinch rolls drive the work piece WP or sheet through the flattener. The movement of the sheet in turn moves or rotates the work rolls 24, 26, 28 as it passes through the rolls.

Several offset work rolls or rollers 24, 26, 28 are positioned within the flattener. Work rolls 24, 26, 28 contact the strip or workpiece WP and bend the strip to the required bend radius. However, rolls 24, 26, 28 themselves do not flex or bend. Back-up rolls 25, 27, 29 positioned adjacent work rolls 24, 26, 28, respectively, are used to prevent the work rolls from bending. If the back-up rolls were not present, the work rolls would bend due to the heavy load of the metal strip pushing against the work roll faces. Also, usage of the back-up rolls allows the work rolls to be made of a smaller diameter than would otherwise be required to resist the bending load on the work roll face. The work rolls 24, 26, 28 are preferably fabricated of an alloy steel or a relatively high hardness.

An entry upper work roll 24 is positioned on the left side of FIG. 1. The work roll 24 preferably has a diameter of about 7 inches, and can travel vertically up to 15 inches between various positions via hydraulic cylinders 40 and roll module mounting beams 42 (see FIG. 4). A back-up roll 25 is positioned above the work roll, and preferably has a diameter of about 7.5 inches. The back-up roll 25 helps keep the work roll 24 in alignment while it receives a heavy bend in the sheet. The back-up roll 25 is also moved by cylinders 40 and beams 42.

A lower work roll 26 is centrally positioned in the flattener, and also has a diameter of about 7 inches. Work roll 26 is offset from upper work roll 24. The lower work roll can travel up to four inches between several positions via hydraulic cylinders such as 40, 44. A back-up roll 27 with a diameter of about 7.5 inches is positioned below the bottom work roll 26. This back-up roll 27 also serves to maintain the lower work roll 26 in alignment while it receives a heavy bend in the sheet. This back-up roll is also moved by hydraulic cylinders 40, 44.

An exit upper work roll 28 is positioned on the right side of FIG. 1. The exit work roll, which is also about 7 inches in diameter, can travel vertically up to 13 inches between several positions via hydraulic cylinders 44 and roll module mounting beams 46. A back-up roll 29, which is about 7.5 inches in diameter, is positioned above the work roll. This back-up roll 29 also serves to maintain the exit work roll 28 in alignment while it receives a heavy bend in the sheet. Back-up roll 29 is also moved by hydraulic cylinders 44 and mounting beams 46.

The center of the entry work roll 24 can be about 2.5 or 3 inches lower than the center of the exit work roll 28. The center of the lower work roll 26 can be about 1.5 to 2 inches lower than the entry work roll to accommodate various thicknesses and bends of metals sheet. However, other orientations of the work rolls are also contemplated by the disclosure.

FIG. 4 is an across machine elevational view of a work roll/back-up roll modules 24, 25, 40, 42. The work roll 24 is shown as a full face work roll (about 7 inches in diameter). The work roll is mounted via bearing supports 33 that are part of a moving frame 35. The back-up roll 25 is also mounted to a cross member 41 of moving frame 35. Back-up roll bearings 37 are also mounted to member 41. There is no independent movement (up or down) between the face of the back-up roll 25 and work roll 24. That is, the work roll 24 and back-up roll 25 relative positions to one another do not change. Attached to the moving frame 35 are the hydraulic cylinders 40 and mounting beams 42. Frame 35 is slidably disposed in vertical slots 39 formed in main frame 10 (FIGS. 1 and 4). Slots 39 allow up and down movement as the hydraulic cylinders extend and retract. The slots prevent movement in other planes or directions. Referring to FIG. 1, roll module 24, 25 is located in milled slots 39 of frame 10. The slots 39 allow up and down (i.e. vertical) movement of the frame but have shoulders to prevent motion in any other direction. The locating slots are in line and below the “open” U cut that accommodates the hydraulic cylinder rod and clevis. The cylinder mounting straddles the open U slot. FIG. 5 is a view through section A-A of FIG. 4 and is a plan view of roll module 24, 25.

Moving frame 35 is essentially the same for the other roll modules 28, 29, 26, 27. Hydraulic cylinder 44 and module support beam 46 are used with rolls 28, 29. For example, FIG. 6 shows a section illustrating the lower work roll 26 and back-up roll 27.

The roll configuration of the flattener is unique in that the three work rolls 24, 26, 28 are used which are typically offset from each other and which receive the sheet material which first passes through a first set of opposed pinch rolls which are closely positioned to form a tight entry for the sheet of material as it enters and exits the pinch rolls. The work rolls include entry and exit rolls and a bottom roll. The back-up rolls 25, 27, 29 add stiffness and compensate for the rolls 24, 26, 28 receiving stresses from the large bends in the sheet.

The work and back-up rolls do not flex but move vertically as described above.

Each of the rolls is positioned to produce a heavy bend radius in the strip end and each is adjustable and programmable by the user to be very precise. The rolls are programmed to be adjustable based on the gauge and strength of the material, and the different bend radii. The programmable positions are determined based on the amount of bending and the radius of the bends, for both positive and negative bends.

FIG. 1 shows the flattener assembly entry and exit work rolls 24, 26, 28 in their operating position and back-up rolls in working position to ride and support the outer diameter of the work rolls.

Referring to FIGS. 4, 5 and 6, it can be seen that the back-up rolls are rotatably mounted on back-up roll mounting brackets 41 that transmit the work roll separating forces to the upper leveler housing. Similarly, the work rolls are rotatably mounted to work roll mounting brackets 33, 43 which in turn are mounted to the flattener movable frame 35. The back-up rolls 25, 27, 29 are shown to be shorter along their longitudinal length than work rolls 24, 26, 28. However, the back-up rolls can be of various lengths.

Referring now to FIG. 4, work rollers 24, 26, 28 are supported at opposite ends of the rollers by beams 43 of frame 35.

Work rolls 24, 26, 28 each are driven or rotate by movement of the workpiece WP between the rolls. Alternatively, the upper work rollers 24, 28 and the lower work rollers 26 can be individually driven by a drive shaft. The back-up rolls and the intermediate back-up rolls are each in supporting contact with a corresponding work roll.

Referring to FIG. 1, a hydraulic cylinder mechanism 40 is mounted to each roll module mounting beam 42 for rolls 24, 25, and a second hydraulic cylinder 44 is mounted to each roll module mounting beam 46 for rolls 28, 29. Actuation of hydraulic cylinders 40 and 44 will cause roll module mounting beams 42 and 46 to shift vertically and/or arcuately to bring the work roll modules 24, 25, 28, 29 to the required roll gap (intermesh) position to cause the sheet to bend around the work rolls as the sheet progresses through the flattener. A similar arrangement can be used for the lower work roll module that consists of work roll 26 and back-up roll 27.

In a typical flattener, the rolls 24, 26, 28 are more evenly spaced and more limited in total roll intermesh capability. In this flattener, work rolls 24, 26, 28 can achieve deeper roll intermesh than a conventional flattener, and the bottom work roll 26 can be raised above the pass line of the sheet. This further increases the sheet bending capability, which results in further reduction of the cross bow defect in the sheet.

The flattener of the present disclosure can accommodate thicknesses of work sheet material WP ranging from about ⅝ inches thick to ¾ inches to 1 inch thick or more. The sheet can be fed through the flattener at a rate of up to 200 feet per minute or more.

As an example, for a 0.5 inch thick steel sheet with a yield strength of 80,000 psi, the metal would be bend or flexed to a radius of between 18 to 30 inches as it presses between work rolls 24, 26, 28. For a 0.25 inch thick sheet of steel with a yield strength of 100,000 psi, the sheet would be bent or flexed to a radius of between 7 to 20 inches as it passes under work roll 24, above work roll 26 and under work roll 28.

The disclosure has been described with reference to a preferred embodiment. Obviously, modifications and alterations may occur to others while reading and understanding the preceding detailed description. It is intended that the disclosure includes all such modifications and alterations insofar as they come within the scope of the preceding description. 

1. A flattening device for flattening a metal workpiece, comprising: a frame; a first tightening assembly at a first end of said frame; a second tightening assembly at a second end of said frame; a series of work rollers interposed between said first tightening assembly and said second tightening assembly, wherein said work rollers comprise a first upper work roller and a second upper work roller, and a third, lower work roller offset and interposed between said first and second upper work rollers; wherein said upper and lower work rollers accommodate bends in said workpiece of up to five yield stresses.
 2. The flattener of claim 1, wherein said first tightening device comprises a first pair of opposed pinch rollers.
 3. The flattener of claim 2, wherein said second tightening device comprises a second pair of opposed pinch rollers.
 4. The flattener of claim 1, further comprising back-up rollers for said first and second upper work rollers and said lower work roller.
 5. The flattener of claim 3, wherein said first pair of opposed pinch rollers and said second pair of opposed pinch rollers are each driven by a motor.
 6. The flattener device of claim 3, wherein said first pair of opposed pinch rollers and said second pair of opposed pinch rollers are vertically shifted by a hydraulic cylinder.
 7. The flattener device of claim 4, wherein said upper and lower work rollers are rotated by movement of said workpiece between said upper and lower work rollers.
 8. The flattener of claim 7, wherein said upper and lower work rollers rotate corresponding back-up rollers.
 9. The flattener of claim 8, wherein said upper and lower work rollers are about 7 inches in diameter.
 10. The flattener of claim 9, wherein said back-up rollers are about 7.5 inches in diameter.
 11. The flattener of claim 10, wherein said pinch rollers are about 12 inches in diameter.
 12. The flattener of claim 1, wherein said upper and lower work rollers are vertically shifted by hydraulic cylinders.
 13. A method of flattening a sheet of metal comprising: providing a flattening device for flattening a metal workpiece, providing a first pair of pinch rollers at a first end of said flattening device; providing a pair of pinch rollers at a second end of said flattening device; providing a series of work rollers interposed between said first pair of pinch rollers and said second pair of pinch rollers, wherein said work rollers comprise a first upper work roller and a second upper work roller, and a third, lower work roller offset and interposed between said first and second upper rollers; and accommodating bends in said sheet of up to five yield stresses as said sheet passes between said upper and lower work rollers.
 14. The method of claim 13, further comprising back-up rollers for said first and second upper work rollers and said lower work roller.
 15. The method of claim 13, further comprising driving said first pair of pinch rollers and said second pair of pinch rollers via a motor.
 16. The method of claim 13, further comprising shifting said first pair of pinch rollers and said second pair of pinch rollers by a hydraulic cylinder.
 17. The method of claim 13, further comprising rotating said upper and lower work rollers by movement of said sheet between said upper and lower work rollers.
 18. The method of claim 13, further comprising shifting said upper and lower work rollers via a hydraulic cylinder. 